I beat him at level 20 with a monk, black mage, white mage, and knight. Make sure you picked up the Blizzaga spell in the cave and your black mage is able to cast level 4 magic. It'll do around 900-1000 damage easily. Once that's used up, resort back to Thundara. I kept my monk and knight using physical attacks and healing items when necessary, with my white mage getting off a couple of Aero spells while using Cure and Cura nearly every turn.